In the initial part of this study, electron spin resonance (ESR) spectra of polystyrene (PS) end-labelled with a nitroxide radical were recorded for both the pure polymer and for (1:1 w/w) blends with polyisoprene (PIP) containing varying amounts of PS-b-PIP triblock copolymer. The spectra reveal that the copolymer migrates to the interphase and that the rate of migration and final packing density of the copolymer in the interphase is dependent on the amount of copolymer in the blends. Further techniques were used to provide complementary data. The second part of this study examined the effect of preparation technique on the interfacial region in immiscible blends of PS with PIP and poly(methyl methacrylate) (PMMA) with poly (2-ethyl hexyl methacrylate) (PEHMA). Blends of these polymer pairs were prepared using the techniques of freeze-drying, co-precipitation and solvent-casting. ESR spectra of these blends recorded as a function of temperature, indicated that the freeze-dried blends exhibited significantly greater interaction between the labelled glassy components and the unlabelled rubbery components than either the co-precipitated or solvent-cast blends. These observations were interpreted on the basis that the average domain sizes of the rubber components within the blends is much smaller in the freeze-dried blends than in blends prepared by other methods, thus producing a relatively greater interphase volume. SEM micrographs of the blends supported this view. The final part of this study examines the local phase behaviour in the miscible polymer pair PS and poly (vinyl methyl ether) (PVME). ESR spectra of both nitroxide end-labelled and in-chain labelled PS were recorded for both the pure polymers and blends of varying composition with PVME. These blends initially form homogeneous, one phase systems followed by phase separation on heating. The results revealed that on phase separation polymer chain-ends tend to accumulate at the newly-formed interphase at the expense of inner chain segments. At temperatures below the cloudpoint slight plasticization of both inner and chain-end labelled PS occurs.